1. Field of the Invention
This invention relates to a torque detecting device of the magnetostriction type used for detecting torque and, more particularly, to an improved torque detecting device which will increase the output of power, decrease the rotative fluctuation of output, and decrease the hysteresis.
2. Description of the Prior Art
It is known that the strain of shaft material is larger at the surface of the shaft rather than at the center when a torque is applied on a measuring shaft such as a rotatable shaft, a fixed shaft and the like.
For example, if torque T is applied clockwise on the measuring shaft against a cross section thereof, a tensile stress +.delta. in the direction slanted on the right side at an angle of 45.degree. to the shaft axis and a compressive stress -.delta. in the direction slanted on the left side at an angle of 45.degree. are generated around the shaft as shown in FIG. 11.
A magnetic substance having a characteristic such that permeability is varied in response to the applied stress on it is called a magnetostriction characteristic, so that the applied stress on a magnetic substance may be measured magnetically using the characteristics of magnetic substances.
There are properties where the permeability increases in the direction of tensile stress in the magnetic substance having the positive magnetostriction and conversely the permeability decreases in the direction of tensile stress in the magnetic substance having the negative magnetostriction.
In the case of detecting the torque applied on a shaft such as a rotatable shaft, a fixed shaft and the like using these properties, it is possible to obtain a large detecting output by concentrating the magnetic flux generated from the excitation coil to the surface of the shaft, i.e., the part in which large strain is produced.
FIGS. 12(a), 12(b) show a conventional torque detecting device. This torque detecting device 51 has a structure wherein a yoke 54 made from a material with high permeability such as a permalloy is disposed in the vicinity of a measuring shaft 52 consisting of magnetic substance having a magnetrostriction effect and spaced a distance 53 from said measuring shaft 52. The yoke 54 is provided with two excitation coils 55 as exciting means forming a magnetic circuit incorporating said measuring shaft 52 into a part of a magnetic path and two detection coils 56 as detecting means detecting the magnetrostrictive force passing through said measuring shaft 52, and the magnetic flux generated from the excitation coils 55 forms a magnetic circuit in which the measuring shaft 52 and the yoke 54 are part of the magnetic path.
In the torque detecting device 51, as shown in FIG. 11 for example, when the torque T is applied clockwise on the measuring shaft 52 against the cross section thereof, the sum of the increasing permeability caused by tensile stress +.phi. in the direction slanted on the right side at the angle of 45.degree. to the shaft axis and decreasing of the permeability caused by compressive stress -.phi. in the direction slanted on the left side at the angle of 45.degree. is used as an output.
More detailed explanations are given as follows. On the occasion of operating the torque detecting device 51 of a magnetostriction type having the aforementioned structure, the magnetic circuit passing through the measuring shaft 52, the gap 53, the yoke 54, the gap 53 and the measuring shaft 52 is formed by charging the excitation coils 55 with electricity. At this time, an induced electromotive force is produced in the coils 56.
In such a state, if the torque T is applied on the measuring shaft 52, the magnetic flux density passing through said magnetic circuit varies as a result of variation of permeability of the measuring shaft 52 due to the magnetostriction effect of the measuring shaft 52 as described above. Correspondingly the variation of the induced electromotive force produced in the detection coils 56 varies and so it is possible to detect the torque applied on said measuring shaft 52 by reading of the said variation of induced electromotive force.
In the case of torque detecting device 51 an output characteristic as shown in FIG. 13 for example is obtained and obtaining this characteristic detection of the torque is carried out by means of a corresponding applied torque detector with detecting output. (Japanese Patent Application No. 60-79238)
However, when the torque applied on a power transmission shaft used as a drive shaft, column shaft and the like is intended to be detected, if the power transmission shaft is used itself as a measuring shaft 52 using a torque detecting device of the magnetostriction type having the structure shown in FIGS. 12a and 12b, the power transmission shaft is generally made from ordinary structural steel, such as JIS, SC, SCr, SCM SNCM and the like, so that the magnetrostriction effect is small and the angle .theta. in the output of the characteristic diagram shown in FIG. 16 is also small. Accordingly, it is impossible to obtain sufficient sensitivity for detection. Further, the problem existed in that it is difficult to detect the torque with accuracy because the width h in said output of the characteristic diagram shown in FIG. 16 becomes large and the hysteresis becomes easy to produce.
Besides, the shaft, such as a rotatable shaft or a fixed shaft provided for measuring the torque, on which a local stress in consequence of machining or heat treatment such as hardening is generally applied, reaches a state where the magnetic wall shifts and sticks due to local stress. Therefore, the permeability is distributed in a state having local anisotropy as shown in FIG. 14 and if the torque is measured in such a state, the output is such that torque T=O kgf-m as shown in FIG. 15 (hereinafter, referred to as "zero-torque") appears as a fluctuation according to the rotative position of the measuring shaft 52. Since the state of distributed permeability in local anisotropy does not vanish but is kept in the range of torque to be measured, the relation between the torque T and output shown in FIG. 15 appears as a fluctuation of output according to the rotative position of the measuring shaft 52 even if the torque T increases, the problem exists that it is impossible to measure the torque T acting on the measuring shaft 52 with accuracy.
Heretofore, the measuring shaft 52 is usually heated to high temperature and subjected to annealing for stress relieving in order to dissolve the state of distributed permeability in local anisotropy as mentioned above in the measuring shaft 52.
However, if the annealing at high temperature is performed on the measuring shaft 52, the material is softened and it becomes easy to cause plastic deformation, and the problem exists that the detecting accuracy of the torque is lowered in consequence of the appearance of hysteresis which is large as shown in FIG. 16.
Thus, on the conventional torque detecting device 51, if the fluctuation of output due to the rotation of the measuring shaft 52 is desired to be eliminated, the hysteresis increases and conversely if the hysteresis is desired to be kept small, the fluctuation of output due to the rotation increases.